1. Field of the Invention
This invention relates to input/output I/O circuits and, more particularly, to I/O signals having programmably controlled polarities.
2. Prior Art
The polarity of a bipolar I/O logic signal is described as the signal level at which the signal is defined to be true. A positive-true signal polarity is one in which a positive signal level represents a true condition, while a zero signal level or a negative signal level represents a false condition. I/O signals are commonly associated with computer or controller systems and the polarity of an I/O signal present in a particular computer or controller system at a particular terminal or integrated-circuit pin is defined to have a predetermined polarity. Previously, a programmer writing computer software for the computer or controller system would need to know and to keep track of whatever polarity had been assigned to a particular I/O terminal or pin. Alternatively, additional inverter circuits are provided as needed as part of the peripheral, or glue logic, circuits which tie operation of the various components of a system together. Keeping track of the polarity of various I/O signals in inconvenient and a source of programming error. Using inverter circuits, particularly at the high current levels associated with I/O signals takes additional circuit space and consumes additional power.
A single-chip controller for use with a variety of different magnetic disk drive models must accommodate a number of different I/O polarities. Solutions in the past have, as described hereinabove, relied upon software tracking and/or use of additional inverter circuits as required.
A circuit for generating an output signal having a selected output polarity is disclosed in the Sievers et al. U.S. Pat. No. 4,670,714, granted June 2, 1987 and titled "Programmable Output Polarity Device." A polarity of an output signal is set by blowing a field programmable fuse. A separate active-high selection line and a separate active-low selection line are provided; each of these selection lines can override the field programmable fuse for test purposes.
For certain I/O terminals of certain integrated circuits, the polarity of those I/O terminals needs to be variable to accommodate particular applications. It is desirable that storage be provided for selective programming and reprogramming the polarity of an I/O terminal.